Pressure-sensitive adhesives (PSA) are used in a variety of applications due to their versatility. For instance, pressure-sensitive adhesives are used for the manufacture of adhesive tapes for packaging or office purposes, yet are also used in the field of medical tapes, plasters, wound or surgical dressings, athletic tapes, or tapes or tabs used in adhering medical devices, such as sensors, electrodes, etc. to the human skin. The later applications are possible since pressure-sensitive adhesives not only adhere to inorganic materials, such as a glass, but also to organic materials, e.g. skin. Further, pressure-sensitive adhesives are known that adhere to wet or moist surfaces, known as “wet-stick” adhesives.
Pressure-sensitive adhesives are known in the art, and usually a high peel strength (adhesion strength) can be obtained. In some applications it is however necessary that the pressure-sensitive adhesive is breathable, i.e. permeable to a certain degree to water vapor. This is particularly important for applications in the medical field, such as for medical tapes, wound dressing or plasters, where a breathable pressure-sensitive adhesive allows for the transportation of moisture from the skin of a wearer, thereby contributing to the wearer's comfort and avoiding or reducing skin irritation. Also in the field of sport tapes such characteristics may be desirable. Conventional pressure-sensitive adhesives are typically not breathable, i.e. have a water vapour transmission rate of 200 g/m2*24 h or less at a thickness corresponding to a coating weight of 30 g/m2 when tested according to the method employed in the Examples of the present invention (UNI-4818-26).
Typically, pressure-sensitive adhesives are based on acrylates, in particular (meth)acrylates. (Meth)acrylates are known for their optical clarity and inherently tacky nature, yet are also chemically robust and resistant to oxidation. (Meth)acrylate-based pressure-sensitive adhesives are typically blends or copolymers prepared from monomers such as (meth)acrylic acid, and esters thereof with non-tertiary alcohols. These monomers are typically copolymerized radically in a solution thereof in an organic solvent. The radical polymerization is typically initiated by including a radical polymerization initiator that decomposes in response to an external stimulus, such as heat, to thereby form radicals that initiate the polymerization reaction. In view of the exothermic nature of the reaction, the polymerization is typically performed by including none or only a small amount of the monomers and the thermal polymerization initiator in a solvent and heating the reaction mixture to thereby form radicals that are capable of initiating the polymerization reaction, and then adding the copolymerizable monomers over time (typically several hours). This allows avoiding a too vigorous reaction. Additionally, modifying the reaction conditions, such the amount of initiator and the monomer feed rate, also allows obtaining polymers having the desired molecular weight.
Alternatively, in the prior art also other kinds of initiating radical polymerization of the monomers were employed, such as use of a compound that decomposes upon irradiation with UV light to thereby form radicals and initiate the polymerization.
Regardless of the type of polymerization initiation, the resulting polymers are then typically essentially linear molecules of high molecular weight, and the polymers have a high viscosity at low temperatures (such as room temperature). In order to process these polymers in the absence of a solvent, such as application to a backing or substrate, the polymers are then usually heated to higher temperatures, thereby reducing their viscosity. Due to their processability in a heated state, these classes of compounds are also known as hotmelt adhesives. Alternatively, the polymers may also be kept in the presence of a solvent, which lowers the viscosity of the overall mixture and which may facilitate the further processing of the polymers. These materials may then be referred to as solvent-based adhesives.
The adhesive properties and further properties of the obtained polymer are strongly influenced by the cohesive properties, the shear resistance and the viscoelastic properties of the polymer. The essentially linear polymer obtained directly after the synthesis as outlined above shows good adhesive properties, yet has only little cohesiveness and shear-resistance. In order to obtain a desired balance of properties, it is often useful to increase the cohesion and shear resistance. This can be achieved by a three-dimensional crosslinking of the essentially linear polymer obtained by the radical synthesis outlined above.
For these reasons, the polymer is typically cross-linked after its application, i.e. on the backing, for instance by further heating or by including a photoinitiator and irradiating the essentially linear polymer on the backing, thereby forming radicals that three-dimensionally crosslink the linear polymers, forming a polymer network.
While therefore a great variety of pressure-sensitive adhesive compositions are known, there still remains a need for improved pressure-sensitive adhesives that can be easily prepared and that provide breathability even in the crosslinked state. For the application in the medical field, the adhesive furthermore must not contain toxic or irritative substances.
From an industrial point of view, it is further preferred that the pressure-sensitive adhesive is a rather homogeneous material and does not cause a phase separation, as could be expected in the case of mixtures of different components. Further, the material should be processable when hot, so that it can be applied onto e.g. a backing or a substrate.
In an attempt to provide pressure-sensitive adhesives for the medical field, U.S. Pat. No. 6,903,151 suggests using an adhesive composition comprising (i) at least one copolymerized monoethylenically unsaturated (meth)acrylic acid ester monomer, wherein the homopolymer thereof has a Tg of less than 10° C., (ii) at least one copolymerized hydrophilic acidic monomer and (iii) at least one non-reactive poly(alkylene oxide)copolymer comprising at least two copolymerized alkylene oxides of which one is hydrophilic (e.g. ethylene oxide) and at least one of which is hydrophobic (e.g. propylene oxide). This adhesive therefore comprises at least three different components, which have to be prepared in advance. Further, the poly(alkylene oxide) copolymer is not reacted with or intimately bound to the other components of the adhesive composition, so that the poly(alkylene oxide)copolymer is likely to leak out from the adhesive composition and to cause a phase separation upon storage.
Compared to such blends of polymers, copolymers have the advantage of being obtainable in a one-step process (polymerization) and after that, are ready-to-use. Blends require an additional mixing step, which is adding cost and can be tedious to achieve perfect homogeneity. Further, in many cases different polymers are not perfectly miscible, causing blending problems and inhomogeneities and differences in performance and characteristics depending on mixing quality. Also, copolymers avoid issues like migration, diffusion, also to the skin, which may cause irritation or poisoning. Copolymers avoid these issues altogether.